For wireless audio streaming, wireless network according to IEEE 802.11 is typically used. According to IEEE 802.11 standard, the power consumption of receiving stations is reduced by switching off the radio unit of the station when the station is not transmitting or receiving data. In particular, the radio units of the receiving stations are turned on after a beacon frame. Wireless communication networks according to IEEE 802.11 standard include frames with a power management field which may be used by the stations in order to change their power management mode.
If a station is in power saving mode, it will only turn on its radio unit at the time it expects a beacon frame sent by the access point AP. The access points may include a traffic indication map TIM in the beacon frame. The traffic indication map TIM comprises information of those clients or receiving stations which are in power saving mode and for which forward traffic has arrived at the access point AP. The receiving station or client will awake when it receives a traffic indication map TIM informing the station that there is data buffered at the access point AP for it. The client or station will then send a power save (PS)-Poll message confirming that the radio is on and is able to receive the data from the access point. Thus, the access point can keep track of which clients are available and which are in power saving mode.
The IEEE 802.11 standard allows not only unicast and broadcast traffic, but also multicast (i.e. group-addressed) traffic. In the case of multicast traffic, however, if at least one of the clients or receiving stations in the group addressed by the group address is in power saving mode, then those data packets destined to this group are repeated after a beacon frame that contains a delivery traffic information map (DTIM) message. The DTIM message is sent in regular intervals, every N beacon frames. It indicates whether or not any group addressed traffic is available. If the beacon frame containing the DTIM message has a multicast or broadcast flag, the receiving stations or clients will receive their data traffic directly after the beacon frame.
FIG. 1a shows a schematic representation of a multicast transmission in a wireless network according to IEEE 802.11 standard. In FIG. 1a, multicast traffic for receiving stations not currently being in power saving mode PS (upper curve) M1 and multicast traffic for receiving stations currently being in power saving mode PS (lower graph) M2 is depicted. In this figure, the rectangles containing “Px” indicate a data packet with the index “x”, whereas the rectangle containing the label “BE” indicates a beacon frame. For simplicity, it is assumed here that each beacon frame contains a DTIM message. If none of the receiving stations or clients in the group address is in power saving mode PS (i.e. all are ready to receive), the data packets in the access point AP which are destined to that group address will be transmitted as soon as they arrive at the access point AP. However, if any receiving station or client registered to the group address is in power saving mode (i.e. not ready to receive), as in the lower curve of FIG. 1a, the data packets must be buffered in the access point AP and will be transmitted after a beacon frame with a DTIM message. Accordingly, the multicast traffic for the receiving stations or clients that are registered to a group address where at least one receiving station or client is in power saving mode is also delayed and transmitted after the DTIM message. In the example of FIG. 1a, the packets P0,P1, . . . ,P4 are transmitted directly if no receiving station or client is in power saving mode, as in the upper curve, but they will be delayed until after the first beacon when at least one receiving station or client is in power saving mode, as in the lower curve. As a result, the packets P0,P1, . . . ,P4 are transmitted as a group P0-4 after the DTIM beacon frame BE. The same happens to the packets P5,P6, . . . ,P9, which are transmitted as a group P5-9 after the second DTIM beacon frame BE.
FIG. 1b discloses a flow chart of a multicast transmission according to FIG. 1a. In step S1, a new multicast packet is received at the access point. In step S2, it is determined if one of the stations is in power saving mode PS. If this is not the case, the flow continues to step S3 where the packet is transmitted without any extra delay. However, if one of the receiving stations is in power saving mode, the flow continues to step S4. Here, the packet is buffered and will be transmitted by the access point AP after a DTIM beacon frame.
Comparing the multicast traffic for the stations with and without power saving mode, it becomes apparent that the minimum delay or latency of a data stream is the interval between DTIM beacon frames if at least one receiving station is in power saving mode. As a result, even receiving stations that are not in power saving mode will experience the same latency if they belong to the same group address as another receiving station that is in power saving mode. Accordingly, the power saving mechanism according to IEEE 802.11 allows power savings, but may increase the delay or latency of the wireless communication. This is often disadvantageous, e.g. for real-time audio data or video data.
U.S. Pat. No. 8,411,608 B2 describes a WiFi multicaster. The traffic from the access point AP towards several receiving stations is received as multicast traffic at the access point and then converted into a number of unicast data streams. This will, however, increase the overall traffic in the wireless network and may lead to additional delay.
U.S. Pat. No. 7,505,447 B2 also shows a conversion of multicast traffic into unicast traffic within a WiFi wireless network.
US2015/0341758A discloses a method and apparatus to ensure that multicasts from access points sent in response to periodic beacons reach a variety of listening clients that may have longer sleep intervals than an interval between beacons. However, this does not reduce the latency for clients that are not in PS mode.
Thus, in addition to the IEEE 802.11 standard, further measures need to be taken in order to improve the wireless group-addressed streaming.